Differences in ligand binding and phosphoinositide turnover between M₁ muscarinic receptor gene transfected cells and mouse and rat brain membranes

The present study describes some unexpected receptor mediated effects of N-methylcarbamylcholine on mouse M₁ muscarinic receptor gene transfected cell line (M₁Y₁) that were not evident from biochemical studies with mouse and rat brain tissue where N-methylcarbamylcholine exhibited only nicotinic properties. Although N-methylcarbamylcholine was devoid of muscarinic properties in mouse and rat brain preparations, as determined by phosphoinositide turnover and inhibition of [³H]QNB binding, it exhibited significant muscarinic characteristics in the transfected M₁Y₁ cell line. At a concentration of 10^-6 M or greater, N-methylcarbamylcholine caused a transient increase in intracellular Ca²⁺ of 50 s duration that was reversible by atropine or pirezepine. The Ca²⁺ transient was not elicited by other nicotinic agents such as nicotine and N,N-dimethylcarbamylcholine, a close analogue of N-methylcarbamylcholine, with comparable affinity for nicotinic receptors and devoid of muscarinic activity. N-Methylcarbamylcholine also stimulated phosphoinositide turnover in M₁Y₁ cells with an estimated EC₅₀ value 10 times greater than that of carbachol, and the effect was blocked by atropine. Both carbachol and N-methylcabamylcholine inhibited [³H]QNB binding in a concentration-dependent manner; however, the IC₅₀ for carbachol was over two orders of magnitude greater than that observed in mouse and rat brain membranes. In considering possible explanations for the differential characteristics of N-methylcarbamylcholine in mouse and rat brain as compared to the transfected M₁Y₁ cells, it was concluded that the difference may be attributable to differences in the receptor-transduction coupling effeciency and the microenvironment of the muscarinic receptors.